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'[PIC:] Car Tacho to PIC input pin protection advis'
2004\09\03@124830 by Kev Pearce \(kevp.com\)
2004\09\03@135338 by hilip Stortz
2004\09\03@181458 by 8859-1?Q?M=E1rcio_Barbiani?=
2004\09\04@104810 by Strshttr
2004\09\05@193039 by Tony Nixon

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Here is a circuit I used.

Tony

Kev Pearce (kevp.com) wrote:

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2004\09\05@202755 by hilip Stortz

picon face
ok, why the 100 ohm resistors on the clamp diodes?  i'd think you'd want
them without the series diodes (the two 18K series resistors should be
more than adequate for current limiting).  why the 4.7k resistor across
the diode?  it seems like you'd want to leave that out unless you are
worried about capacitance?  and why the 10k resistor to ground?  is that
also for the capacitance (which should be mostly from the capacitor). also, how are you protecting the 5V supply?  the transistor would also
appear to be somewhat unnecessary if you were using a comparator input,
though i see it could be used for level shifting.  a lot of questions i
know, i'm just trying to understand the logic behind most of that
design, which appears to use extra components (like 2 18K resistors
instead of one around 40K, i can't see power dissipation being a
problem).  i'd also use 1n4148 or 1n914 switching diodes instead of the
slow 1n4004 rectifiers (which could also be anything from 1n4001 to
1n4004 diodes i'd think?) as you want fast diodes for limiting
transients, and the current rating should be more than adequate.  Please
explain, i'm sure you have reasons.

Tony Nixon wrote:
>
> Here is a circuit I used.
>
> Tony
-------

-- Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist. Then they came for the Jews, and I didn't speak up because I wasn't a
Jew. Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist. Then they came for the Catholics, and I didn't speak up because I was a
Protestant. Then they came for me, and by that time no one was left to speak up."
-- Martin Niemöller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.

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2004\09\06@104918 by Dominic Stratten

picon face
100 ohm resistors on the clamp diodes are in case the numpty installing the
unit connects it up with the polarity reversed. The 1N4004 diodes are there
as they are rated for 400 volts (this is easily achieved in the ignition
circuit).

The two 18k resistors are necessary because they each have a breakdown
voltage of 200 volts. One 40k resistor may not provide enough insulation for
the 300V+ that appears on modern ignition systems (back EMF).

I don’t know about the 4.7k across the diode but to be honest with you I've
used this circuit a number of times and it works without any problems at
all. I personally use an interrupt pin rather than a comparitor input (this
circuit has been around since the days of the 16F84).

Just my personal experiences with the circuit.

Dom

{Original Message removed}

2004\09\07@130228 by hilip Stortz

picon face
sounds good, but i'd still use 1n914/1n4148 diodes, the reverse polarity
problem will only be with the 12V system.  one diode should conduct long
before that point reaches 400V even if one resistor fails, if they both
fail other components would fry even if the diodes didn't, i.e. the
transistor wouldn't take a 400V dump, and i think you want the fastest
diodes you can reasonably get to protect the other components, slow
diodes allow the voltage to rise to a higher level before it is clamped
to a supply rail.  i understand using multiple resistors in series in
this case, though with the numbers you gave i'd feel more comfortable
using 3 or 4 resistors in series, and i'd definitely add the diode
clamps to high side of the one farthest from the points.  (i've built 2
hv dividers using multiple 10M ohm 1/4 resistors for an analog volt
meter, i think one 60 of them, and i potted them to cheat a little on
the voltage rating, i wouldn't have used it in something permanently
connected).
all things considered, i think the previous mention of opto isolation
would be a very fine idea, saving cost, parts, and providing better
safety for the rest of the system.  note that i'd put clamp diodes on
the input to the isolator, which would probably still require the
multiple resistors for current limit and possibly a small cap to reduce
the rise time of sharp transients and give the protection diodes time to
react.  using your circuit, i think i'd put the small cap where you have
the diodes clamping to the supply rails for the same reason, i.e. the
diodes don't switch instantly, so the maximum voltage at that point is
the dv/dt * (diode switch time), where as with the capacitor you've
slowed the voltage rise and hence reduced the peak voltage.

i'm sure your circuit works well, and i have a better idea of why you
did it that way, which i appreciate.  that's why i asked in the first
place.  having learned the reasons, i do have a few ideas that might
make it better.  i appreciate seeing how others have solved problems,
it's very educational and always useful in the long run.  i'd definitely
field test any modifications to your circuit before i used it in a
commercial product, there are heat and vibration issues which can
complicate things.

given the voltage spikes, i might also consider an inductive pickup for
this as it lets you easily limit the maximum energy available to
potentially do damage.  if you could find one, one of the old
optoisolators that used a neon lamp might be ideal, if the lamp in them
is potted to handle the vibration.  of course you could make one of
those, but protecting the glass from vibration on the lamp leads would
be a consideration.  another very good possibility would be to use a
mosfet with a suitably high voltage rating, possibly with a voltage
divider and clamping.  should i ever build such a circuit i'm sure i'll
have fun trying different alternatives just to learn what fails and what doesn't.

Dominic Stratten wrote:
>
>
> 100 ohm resistors on the clamp diodes are in case the numpty installing the
> unit connects it up with the polarity reversed. The 1N4004 diodes are there
> as they are rated for 400 volts (this is easily achieved in the ignition
> circuit).
-------

-- Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist. Then they came for the Jews, and I didn't speak up because I wasn't a
Jew. Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist. Then they came for the Catholics, and I didn't speak up because I was a
Protestant. Then they came for me, and by that time no one was left to speak up."
-- Martin Niemöller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.

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2004\09\07@162610 by olin_piclist
face picon face
Philip Stortz wrote:
> i think you want the fastest
> diodes you can reasonably get to protect the other components, slow
> diodes allow the voltage to rise to a higher level before it is
> clamped
> to a supply rail.

This is a common misconception.  Diode speed ratings generally refer to the
reverse recovery time (on to off), not off to on.  The off to on time of
silicon diodes is not an issue in the vast majority of cases.  Many data
sheets don't even specify it.


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2004\09\07@165458 by hilip Stortz

picon face
yes, i know it isn't usually specified, and the on to off time is
important for efficiency usually, but surely 1n4004 diodes turn on more
slowly then 1n4148, the switching speed is related to the junction size
and barrier thickness, and other parameters of the diode isn't it? surely diodes that switch off fast switch on fast, and diodes that
switch off slowly switch on slowly.  there's the junction capacitance if
nothing else.  i know transistors have similar speeds in turning on and
off, surely diodes behave in a similar fashion?  if not, it would be a
terribly useful loop hole.

Olin Lathrop wrote:
{Quote hidden}

------

-- Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist. Then they came for the Jews, and I didn't speak up because I wasn't a
Jew. Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist. Then they came for the Catholics, and I didn't speak up because I was a
Protestant. Then they came for me, and by that time no one was left to speak up."
-- Martin Niemöller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.

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http://mailman.mit.edu/mailman/listinfo/piclist

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